Smoking articles, particularly cigarettes, generally comprise a tobacco rod of shredded tobacco (usually, in cut filler form) surrounded by a paper wrapper, and a cylindrical filter aligned in an end-to-end relationship with the tobacco rod. Typically, the filter includes a plug of cellulose acetate tow attached to the tobacco rod by tipping paper. Ventilation of mainstream smoke is achieved with a row or rows of perforations about a location along the filter. Such ventilation provides dilution of drawn mainstream smoke with ambient air to reduce the delivery of tar.
Particulate efficiency of a filter is typically resolved as the level of tar into a filter minus tar level out of the filter divided by the tar level into the filter. Ventilation tends to lower particulate efficiency of a filter.
Upon lighting a cigarette, a smoker draws mainstream smoke from the coal at the lit end of the cigarette. The drawn cigarette smoke first enters the upstream end portion of the filter and then passes through the downstream portion adjacent the buccal (mouth) end of the cigarette.
Certain cigarettes have filter segments which incorporate adsorbent materials such as activated carbon, and examples of such are described in U.S. Pat. No. 2,881,770 to Tovey; U.S. Pat. No. 3,353,543 to Sproull et al.; U.S. Pat. No. 3,101,723 to Seligman et al.; and U.S. Pat No. 4,481,958 to Ranier et al. Certain commercially available filters have particles or granules of carbon (e.g., an activated carbon material) alone or dispersed within a cellulose acetate tow; other commercially available filters have carbon threads dispersed therein; while still other commercially available filters have so-called “plug-space-plug”, “cavity filter” or “triple filter” designs. Examples of commercially available filters are SCS IV Dual Solid Charcoal Filter and Triple Solid Charcoal Filter from Filtrona International, Ltd.; Triple Cavity Filter from Baumgartner; and ACT from Filtrona International, Ltd. See also, Clarke et al., World Tobacco, p.55 (November 1992). Detailed discussion of the properties and composition of cigarettes and filters is found in U.S. Pat. Nos. 5,404,890 and 5,568,819 to Gentry et al, the disclosures of which are hereby incorporated by reference.
Typical of prior practices with “plug-space-plug” styled cigarettes has been heretofore to locate ventilation at a location along the bed of adsorbent contained in the space, so as to achieve sufficient spacing of the ventilation holes from the buccal end of the filter. In so doing, the lips of the smoker would not occlude the ventilation holes. Such placement, however, tended to lower the filtration effectiveness of the adsorbent, because it tended to increase the velocity of the mainstream smoke in at least a portion of the absorbent bed.
Various annular configurations of filters having carbon-bearing annular filter regions are disclosed in the prior art. For example, European Patent Application No. 579,410 shows a number of cigarette embodiments having an annular carbon-bearing region surrounding either porous filtration material or an empty tubular cavity formed by a vapor phase porous membrane. Similarly, U.S. Pat. No. 3,894,545 to Crellin et al. shows various configurations of annular carbon-bearing regions surrounding a vapor phase porous membrane or a rod of carbon-bearing material surrounded by a vapor phase porous membrane.
Cigarette filter elements which incorporate carbon have the ability to remove constituents of mainstream smoke which passes therethrough. In particular, activated carbon has the propensity to reduce the levels of certain gas phase components present in the mainstream smoke, resulting in a change in the organoleptic properties of that smoke.
Despite these advantages of carbon bearing filters, they are not so widely employed. It has been found that mainstream smoke from carbon filters tend to have a flavor note that is contrary to consumer preferences, and that therefore their employment in commercially offered cigarettes has not been heretofore widespread.
It would be desirable to provide a cigarette having a cigarette filter incorporating carbon and/or other materials capable of absorbing and/or adsorbing gas phase components present in mainstream cigarette smoke, while providing favorable absorption/adsorption, dilution and drawing characteristics, and adding flavor to the filtered smoke so as to enhance consumer acceptability.
Furthermore, it would be desirable to provide such a filter with desirable residence time in the adsorbent/absorbent-containing region while simultaneously achieving a pressure drop downstream of the dilution region and the adsorbent/absorbent so as to provide acceptable drawing characteristics of puffs of smoke having reduced gas phase components but with acceptable taste and resistance-to-draw.